1. Field of the Invention
The present invention relates to a material for a sintering appliance which is used for sintering a functional ceramic and a ceramic powder.
2. Description of the Related Art
For maintaining the prescribed characteristics of sintered products, it is required that the components of the material for a sintering appliance which is used for sintering a functional ceramic, e.g., electrical parts, should not react with the member of the substance to be sintered, or the components of the substance to be sintered should not be absorbed in the material for a sintering appliance during sintering. For that sake, in many cases, a dense or low reactive film layer is formed on the surface of the material for a sintering appliance.
For forming such a film layer on the surface of the base material, various methods are adopted, e.g., a method comprising casting or spraying slurried film components on the surface of the molded, or molded and further sintered, base material, then sintering; a method comprising fixing a film layer on the surface of the molded, or molded and further sintered, base material by means of an adhesive material, e.g., a vitreous material; and a method comprising forming a film layer by flame-coating and at the same time baking it on the surface of the base material previously sintered.
Of these, a method by flame-coating is particularly preferred because a dense coated film can be obtained on the surface of the base material and abrasion resistance of the coated film per se is high. Therefore, when the film layer of the material is formed by flame-coating, the component of the substance to be sintered (e.g., Pb, Bi, Na, K, etc.) hardly penetrate the base material through the film layer to deteriorate the base material and generate cracks or warps.
Alumina and zirconia are used in general as the material of the film in a flame-coating method taking the cost, the reactivity with the substance to be sintered, and the use life of the coated film into consideration. In particular, when the reactivity resistance is important, zirconia is used.
Further, for preventing peeling-off attributable to difference of thermal expansion between the base material and the flame-coated zirconia film from occurring, zirconia is often used as stabilized zirconia or partially stabilized zirconia in a zirconia flame-coated film. In view of reactivity resistance and cost saving, yttrium oxide (Y.sub.2 O.sub.3) and calcium oxide (CaO) are generally used as a stabilizer. However, in recent years, higher quality of sintered functional ceramic and ceramic powder has been further demanded. Therefore, in the sintering of a certain soft ferrite and dielectric ceramic and a special ceramic powder, the content of CaO in a zirconia film must be increased to ensure the characteristics of a sintered product, as a result, a part of components of the substance to be sintered happens to react with the base material.
However, if the content of CaO in a zirconia film is further increased, base materials for use are limited correspondingly while such base materials have a drawback on use life such that cracks are generated due to properties of base materials per se.
With respect to this problem, JP-B-3-77652 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a technique comprising flame-coating stabilized zirconia containing from 4 to 31% by weight of CaO on an alumina-silica base material containing 85% by weight or more of alumina. However, JP-B-77652 discloses that if thermal expansion of the base material and the flame-coated film are not approximated, the coated film is liable to peel off, for example, if the difference of thermal expansion between both at 1,200.degree. C. is 0.06% or more, the coated film peels off.
Further according to this technique, as the more the content of the stabilizer (CaO), the more increases the thermal expansion of the stabilized zirconia, it is necessary to use the alumina-silica base material having a large alumina content and this technique limits the content of alumina to 85% by weight or more.
Accordingly, if a flame-coated film having a large content of CaO is used, the alumina content of the base material has to be increased correspondingly. In such a case, however, the base material itself is liable to crack and the use life of the material of the appliance itself becomes low, although the characteristics of the flame-coated film can be obtained. Further, there is a problem such that the coated film of the stabilized zirconia in some cases lets CaO free with the destabilization by thermal hysteresis and the free CaO reacts with sintered products.
JP-B-4-568 discloses a technique comprising flame-coating stabilized zirconia having a CaO content of from 4 to 15% by weight on the surface of an alumina base material so as to gradually decrease the content of CaO. However, the thus-obtained material of the appliance has less content of CaO of the surface side, which cannot be effectively used for sintering dielectric ceramic and a special ceramic powder unless the content of CaO in zirconia film is increased.
Further, JP-B-4-21330 discloses a technique in which a flame-coated alumina layer or a sintered alumina layer intervenes between an alumina base material and a zirconia flame-coated layer to improve adhesion of the base material with the zirconia layer by lessening difference of thermal expansion between the base material and the zirconia film layer. However, this technique requires two or more steps of flame-coating, which is expensive. Moreover, also in this case, stabilized zirconia or partially stabilized zirconia containing 8% by weight or less of CaO is used for coating, which cannot be used for sintering a special powder, etc. as above.